CPC7593 Datasheet, PDF (20/24 Pages) Clare, Inc. – Line Card Access Switch

INTEGRATED CIRCUITS DIVISION
2.7 Power Supplies
Both a +5 V supply and battery voltage are connected
to the CPC7593. Switch state control is powered
exclusively by the +5 V supply. As a result, the
CPC7593 exhibits extremely low power consumption
during active and idle states.
Although battery power is not used for switch control, it
is required to supply trigger current for the integrated
internal protection circuitry SCR during fault
conditions. This integrated SCR is designed to
activate whenever the voltage at T_BAT or R_BAT drops 2
to 4 V below the applied voltage on the V_BAT pin.
Because the battery supply at this pin is required to
source trigger current during negative overvoltage
fault conditions at tip and ring, it is important that the
net supplying this current be a low impedance path for
high speed transients such as lightning. This will
permit trigger currents to flow enabling the SCR to
activate and thereby prevent a fault induced negative
overvoltage event at the T_BAT or R_BAT nodes.
2.8 Battery Voltage Monitor
The CPC7593 also uses the V_BAT pin to monitor
battery voltage. If the system battery voltage is lost,
the CPC7593 immediately enters the all-off state. It
remains in this state until the system battery voltage is
restored. The device also enters the all-off state if the
battery voltage rises more positive than about –10 V
and remains in the all-off state until the battery voltage
drops below –15 V. This battery monitor feature draws
a small current from the battery (less than 1 A
typical) and will add slightly to the device’s overall
power dissipation.
This monitor function performs properly if the
CPC7593 and SLIC share a common battery supply
origin. Otherwise, if battery is lost to the CPC7593 but
not to the SLIC, then the V_BAT pin will be internally
biased by the potential applied at the T_BAT or R_BAT
pins via the internal protection circuitry SCR trigger
current path.
CPC7593
2.9 Protection
2.9.1 Diode Bridge/SCR
The CPC7593 uses a combination of current limited
break switches, a diode bridge/SCR clamping circuit,
and a thermal shutdown mechanism to protect the
SLIC device or other associated circuitry from damage
during line transient events such as lightning. During a
positive transient condition, the fault current is
conducted through the diode bridge to ground via
F_GND. Voltage is clamped to a diode drop above
ground. During a negative transient of 2-4 volts more
negative than the voltage source at V_BAT, the SCR
conducts and faults are shunted to F_GND via the SCR
or the diode bridge.
In order for the SCR to crowbar or foldback, the SCR’s
on-voltage (see “Protection Circuitry Electrical
Specifications” on page 13) must be less than the
applied voltage at the V_BAT pin. If the V_BAT voltage is
less negative than the SCR on-voltage, or if the V_BAT
supply is unable to source the trigger current, the SCR
will not crowbar.
For power induction or power-cross fault conditions,
the positive cycle of the transient is clamped to a diode
drop above ground and the fault current is directed to
ground. The negative cycle of the transient will cause
the SCR to conduct when the voltage exceeds the
V_BAT reference voltage by two to four volts, steering
the fault current to ground.
Note: Neither the CPC7593xB or the CPC7593xD
contains the protection SCR but instead uses a diode
bridge to clamp both polarities of a fault transient.
These diodes direct the negative potential’s fault
current to the V_BAT pin.
2.9.2 Current Limiting function
If a lightning strike transient occurs when the device is
in the talk state, the current is passed along the line to
the integrated protection circuitry and restricted by the
dynamic current limit response of the active switches.
During the talk state when a 1000V 10x1000 S pulse
(GR-1089-CORE lightning) is applied to the line
though a properly clamped external protector, the
current seen at T_LINE or R_LINE will be a pulse with a
typical magnitude of 2.5 A and a duration of less than
0.5 s.
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